Alanine aminotransferase (hereinafter referred to as ALT) is an enzyme abundantly distributed in the heart or liver. Because ALT is released to blood during a disease, the measurement of an ALT activity in a body fluid such as urine or blood is important as a marker for a diagnosis of a heart or liver disease or observation after treatment thereof.
In a method commonly used for measuring the ALT activity, pyruvic acid, which is generated from L-alanine and 2-oxoglutaric acid as substrates by ALT, is changed to lactic acid by lactate dehydrogenase (hereinafter referred to as LD), and a decreased amount of reduced nicotinamide adenine dinucleotide (hereinafter referred to as NADH) coexisting is measured at the wavelength of approximately 340 nm.
The reaction formulae are as follows:

In the above formula, NAD means oxidized nicotinamide adenine dinucleotide.
As previously described, the measurement of ALT activity is important as a marker for a diagnosis of a heart or liver disease or observation after treatment thereof, and is carried out internationally. However, because various methods for measuring the ALT activity are known and used, different values obtained in accordance with different facilities or persons lack compatibility, and thus there is concern that problems in clinical diagnosis may arise. Therefore, recommendations in which reaction principles, the composition of reagents, concentrations of reagents, and the like are prescribed are suggested in each country, and the ALT activity is thus measured in each facility, to ensure compatibility with values measured in accordance with the recommendations.
In Japan, the Japan Society of Clinical Chemistry (JSCC) published in 1989 a recommendation for measuring the ALT activity [JSCC: Recommendation for measuring enzyme activities in human serum-alanine aminotransferase-(1989-08-30), Japanese Journal of Clinical Chemistry, 18(4), 250–262 (1989)].
The recommendation published by JSCC is a method in which common enzyme activities are measured under optimal conditions, which may be shared by technical levels in a clinical laboratory or the like, to ensure the compatibility of measuring values, and thus it cannot be generally used as a daily test. Therefore, manufacturers of reagents for clinical laboratory test supply reagents capable of ensuring compatibility between measuring values and the JSCC recommendation, and have conducted intensive studies into the provision of stable and low-cost reagents to obtain accurate and precise measuring values and the compatibility between measuring values and the JSCC recommendation.
In facilities such as a clinical laboratory where the ALT activity is measured, reagents for measuring the ALT activity, which are supplied by the manufacturers of reagents for clinical laboratory test and ensure the compatibility between measuring values and the JSCC recommendation, are generally used as a daily test. However, because samples from a living body are mixtures of various components and reagents for measuring the ALT activity are also mixtures of various components, it is very difficult to develop a reagent which is stable and not affected by impurities, for measuring the ALT activity.
Particularly, in an automatic analyzer in which only the setting of measuring reagents and the setting of measuring conditions are needed, measurement is often carried out after keeping reagents open for several weeks. In this case, the reagents for measuring the ALT activity absorb carbon dioxide in air, and then pH in the reagents followed by the reagent blank reaction changes, to produce a problem, i.e., errors in the measuring values of ALT activity obtained.
Therefore, the object of the present invention is to provide a reagent for measuring the ALT activity which can suppress an increase in the reagent blank reaction, i.e., an increase in the initial absorbance.